A large number of endogenous peptides found in various regions of the central nervous system exhibit opioid activity. This activity is elicited by compounds, known as opiates, which are described as substances which bind to opioid receptors. Concomitant with the multitude of peptides which demonstrate opioid activity was the pharmacological discovery of the existence of multiple opioid receptors. See W. Martin et al., J. Pharmacol. Exp. Ther., 197, p. 517 (1975); and J. Lord et al., Nature (London), 257, p. 495 (1977). Three different types of receptors have been identified. The first, .delta., shows a differentiating affinity for enkephalin. The second, .mu., shows an enhanced selectivity to morphine and other poly-cyclic alkaloids. The third, .kappa., exhibits equal affinity for either group of these ligands and preferential affinity for dynorphin. .beta.-endorphin appears to bind equally to the .mu. and .delta. receptors. In general, the .mu. receptors seem to be more involved with analgesic effects, whereas the .delta. receptors appear to deal with behavioral effects, although the .delta. and .kappa. receptors may also mediate analgesia.
The major putative function for opiates is their role in alleviating pain. Other areas where opiates are well-suited for use in treatment are conditions relating to gastro-intestinal disorders (diarrhea), schizophrenia, obesity, blood pressure, convulsions, and where seizure induction may alleviate earning or memory loss.
To date, opiates, opioid peptides, and analogs thereof, have either failed to demonstrate, or have demonstrated a limited degree of specificity and selectivity for the receptor or receptors to which they may bind. The less selective and specific an opiate may be, the greater the chance that increased side effects from the administration of the material will be observed. Since each receptor certain effects are symptomatic when each receptor is triggered. However, when an opiate activates more than one receptor, the biological response profile for each receptor is affected, thereby potentiating a spectrum of side effects which may or may not be adverse. Such adverse side effects include heaviness of the limbs, flush or pale complexion, clogged nasal and sinus passages, dizziness, depression, and the like.
Conventional opiate peptides have been chosen from among enkephalin, endorphin, and dynorphin analogs. Other known opiates are poly-cyclic alkaloid structures such as morphine, naloxone, and levorphanol. It is generally recognized that the degree of complementary affinity between the structural characteristics of an opiate receptor and the ligand which functions as an opiate or opioid peptide, is significant to the determination of receptor selectivity and specificity. However, recent reports indicate that specific interactions of opioid peptides with various membrane compartments may also contribute to the ability of these opiates to selectively interact with specific receptors. See R. Schwyser, Biochemistry, 25, p. 6336 (1986).
The term "specificity" refers to the particular or definitive preference of an opiate or opioid peptide for one opioid receptor over another. The term "selectivity" refers to the ability of an opiate or opioid peptide to discriminate among several opioid receptors. For instance, the ratio of binding inhibition constants, K.sub.i.sup..delta. /K.sub.i.sup..mu., is a value that may be used to measure selectivity. This ratio represents the relationship of the affinities for binding to the .delta. and .mu. receptor. A higher value for this ratio indicates a greater preference of ligand to bind with the .mu. receptor over the .delta. receptor. One conventional opioid peptide analog, H-Tyr-D-Ala-Gly-Phe(NMe)-Gly-ol (DAGO), has been understood to be the most .mu. selective opioid peptide analog known to date. This peptide has recorded a K.sub.i.sup..delta. /K.sub.i.sup..mu. value of 1050. Leu-enkephalin, on the other hand, has recorded a K.sub.i.sup..delta. /K.sub.i.sup..mu. of 0.2. This fractional value reflects a pronounced affinity for the .delta. receptor over the .mu. receptor. Thus, it would be desirable for an opiate or opioid peptide to demonstrate an increased value for this binding ratio, thereby generating a greater preference for .mu. receptor binding and enhancing the peripherally mediated analgesic effects related to these opiates and opioid peptides.
The class of compounds commonly referred to as analgesics are normally quite hydrophobic and thus are extremely well-suited to permeate lipid membranes, such as the blood-brain barrier. Due to this physical capability, these compounds tend to bind with opioid receptors of the central nervous system in the brain, although not necessarily the same ones. This binding may generate similar side effects to those attributed to conventional opiates, opioid peptide and analogs, although the problems are likely to be of a greater magnitude.
Similarly, the hydrophobic character of conventional opiates and opioid peptides tends to delay their rate of passage through the system. Much effort has been placed on improving the adsorption properties of these compounds and decreasing their capability to transcend the blood-brain barrier. However, since their affinity and solubility is greater for lipid materials than for aqueous fluids of physiological pH, these compounds tend to lodge themselves in the organs and fatty tissue of the body. They tend to remain therein for extended periods of time rather than being excreted from the system. The long term toxicity of many of these compounds is generally uncertain, however any prolonged exposure is likely not to be beneficial.
Recent advances in the field of opioid peptides have been directed towards the stabilization of these peptides against enzymatic or hydrolytic degradation. It would be extremely valuable to stabilize these peptides from proleolytic enzymes in order to improve their pharmocokinetic properties. Enhanced resistance to enzymatic degradation would increase the usefulness of these opioid peptides as therapeutic agents. However, since they only exhibit short half lives in vivo, large amounts of such peptides must typically be administered to a subject in order to achieve the desired effect. Alternatively, smaller quantities may be prescribed to an individual, but more frequent dosages would be required to achieve the same level of potency.
It is desirable that a peptide meet several general criteria in order to be considered for any pharmacological interest. First, a peptide should be resilient to proteolytic degradation. Second, a peptide should elicit an enhanced biological response. Third, a peptide must be safe for human consumption. Fourth, a peptide should be capable of being produced in quantities large enough to perform clinical studies respecting its toxicity and later for commercialization. In the present case, less lipid solubility and greater aqueous solubility are also desirable properties for the peptide to possess, to prevent permeation through the blood-brain interface and to permit rapid excretion of any excess administered peptide and its metabolites. Further, it would be desirable for a peptide to elicit selective and specific receptor binding potential, in order to minimize potential side effects resulting from its ingestion.
Regarding suitable production quantities discussed above, several factors are worthy of consideration. The method of preparing the peptide should preferably be simple, efficient, and economical. That is, the reaction scheme of the method should contain few steps, afford high overall yields, and allow a minimum amount of by-product formation. Moreover, the scheme should preferably utilize inexpensive reagents and materials. In this respect, inexpensive and reproduceable diagnostic and analytical methods should be incorporated into the production scheme where feasible. Further, the method should ensure the optical integrity of the peptide by avoiding reagents that will tend to racemize the reactants and products. A racemic mixture of the desired peptide is not likely to fully exhibit the desired pharmacological response.
The optical integrity of a compound relates to its ability to rotate light. This ability is measured by an instrument known as a polarimeter, which utilizes a zero point reference to base its reading. The degree to which a chemically pure material rotates light indicates its relative optical purity. That is, a material may be chemically pure while being optically inactive or racemic. The amount of activity that is observed from a material is often dependant upon its optical purity. Two enantiomers, although possessing identical chemical formulae, may have completely different biological activities. It is common in medicinal applications for a compound of one optical configuration to exhibit activity and usefulness, while its optical rotamer or complimentary enantiomer demonstrates a different activity or is wholly inert. Thus, where optical configuration is important, optical purity, as well as chemical purity is an important concern.
Conventional opiates and opioid peptides exhibit either no specific and selective binding preference for opioid receptors, or limited specific and selective binding preference for those receptors. Moreover, their hydrophobicity and enhanced lipid solubility have permitted them to cross the blood-brain interface to some extent. Further, the combined effects of these two phenomena not only establish the potential for side effects, particularly adverse side effects, and those side effects are likely to be varied and intensified due to the centrally mediated interactions of the opiate or opioid peptides.
Notwithstanding the efforts placed against these concerns heretofore, there is a need for peptides which exhibit a preference for a specific opioid receptor, especially the .mu. receptor. It would be desirable to design peptides of less lipid solubility than that of conventional opiates or opioid peptides, so that the blood-brain barrier would be impermeable to these peptides. Further, peptides of high polar character would normally possess a greater tendency to become solublized in aqueous fluids of physiological pH, thereby facilitating their excretion and the passing of their metabolites from the system. It would also be desirable to synthesize these peptides in a simple, efficient, and economical manner to facilitate the preparation of suitable quantities for toxicological studies and commercial supplies while retaining the optical integrity of the desired materials.